This invention relates to NMR spectroscopy and, more specifically, NMR spectroscopy of fluids contained in porous materials.
Coring of an earth formation is a routine procedure in oil production operations. A core is obtained in order to retrieve a small section of the reservoir formation that may be used to measure rock properties and determine insitu oil saturation.
Unfortunately, the coring process subjects the core to high fluid velocities and high pressures that disturb the core and any insitu fluids within the core. Solid particles such as barite in the drilling mud often invade deeply into the core. In addition, drilling fluids from either a water-based mud or an oil-based mud can invade the core and flush out some of the insitu oil saturation.
The standard analysis technique used in the oil industry for determining oil saturation in cores to be cut small 1" outside diameter plugs every foot of the core and extract the plugs using the Dean-Stark extraction technique. This is a laborious and expensive procedure. Moreover, this procedure will often sample those parts of the core that have been seriously flushed by mud filtrate and have had their oil saturations reduced. Thus, the oil saturations for core plugs are typically understood to be the minimum possible oil saturation in the reservoir, while the true unflushed oil saturation in the reservoir is often significantly higher.
Sponge coring is an attractive alternative to the blowdown losses of conventional coring, without the expense and low recovery associated with pressure core. A sponge core barrel traps the oil expelled from the core in an oil-wet, high porosity polyurethane sponge surrounding the core. At the surface, cored sections are stored in completion brine or frozen and then transported to the laboratory to determine the quantity of oil trapped in the sponge.
The present method of determining the fluid saturations in the sponge is by solvent extraction. Typically two days are required for the extraction process on each one foot section of sponge. The solvent must then be separated from the extracted oil and water volumes. Finally, the oil volume must be corrected for non-reacted components of the polyurethane which are removed along with the oil in the extraction.
These and other limitations and disadvantages are overcome by the present invention, however, and methods are provided for obtaining insitu oil saturations from core.